Cardiac arrhythmia affects millions of people worldwide and is broadly defined as an abnormal or irregular heartbeat that may involve changes in heart rhythm, producing an uneven heartbeat, or heart rates, causing a very slow or very fast heartbeat. Common types of arrhythmias, explained in further detail below, include bradyarrhythmias and tachyarrhythmias, both being typically ventricular or supraventricular in origin.
Bradyarrhythmias are slow heart rhythms (e.g., less than 60 beats per minute) that may result from a diseased or failing sinoatrial (SA) node, atrioventricular (AV) node, HIS-Purkinje, or bundle branch system, as explained in further detail below. Ventricular arrhythmias are arrhythmias that begin in the lower chambers of the heart. In contrast, supraventricular arrhythmias are arrhythmias that originate above the ventricles of the heart, such as the upper chambers (i.e., atria) or the middle region (e.g., AV node or the beginning of the HIS-Purkinje system). Ventricular and supraventricular arrhythmias are generally characterized by accelerated rates (e.g., more than 100 beats per minute) that exceed what is considered normal heartbeat rhythms (e.g., between 60 and 100 beats per minute).
The most common type of supraventricular arrhythmia is atrial fibrillation, with incidence of more than a quarter-million cases each year in the U.S. alone, and a prevalence of nearly 2.0 per 1000 US patient-years, To better understand the mechanism and characteristics of atrial fibrillation, a general understanding of the mechanical and electrical activity of the heart is helpful. For this purpose, attention is directed to FIG. 1.
FIG. 1 depicts a cross-sectional diagram of a normal, healthy heart 10. The heart 10 is a four-chamber, double-sided pump made of muscle tissue that contracts when subjected to electrical stimulation. The electrical stimulation that produces a heartbeat originates in the SA node 12, located at the junction of the superior vena cava 14 with the right atrium 16, and spreads radially through the atria causing the muscle of the heart's upper chambers to contract and pump blood to the ventricles. From the atria, the electrical signal then converges on the AV node 18, located in the right posterior portion of the interatrial septum. The impulse from the AV node 18 then passes to the bundle of HIS 20, which branches at the top of the interventricular septum 22 and runs subendocardially down either side of the septum, and travels through the bundle branches 24. The signal then passes to the Purkinje system 26 and finally to the ventricular muscle causing the lower chambers of the heart to contract and pump blood to the lungs and the rest of the body After contraction of the lower chambers, the sinus node initiates the next rhythm or heart beat and the entire cycle is repeated. In general, it is rate of discharge from the SA node 12 (also referred to as the normal cardiac pacemaker) that determines the rate at which the heart 10 beats.
This synchrony of contraction between the atria and ventricles produces a normal heartbeat. In its broadest sense, atrial fibrillation (AF) represents a loss of synchrony whereby the atria quiver (beating at a rate of about 600 beats per minute) instead of beating or contracting effectively. The loss of atrial contraction and conduction of electrical signals from the atria to the ventricles often cause blood to pool and clot in the atria, and especially in the atrial appendages. If the clot becomes dislodged from the atrium, it can travel through the bloodstream and create a blockage in a vessel that supplies blood to the brain, resulting in stroke. It is estimated that fifteen percent of all strokes occur in people with AF, which translates to about 90,000 strokes each year in the United States alone.
Conventional therapy or treatment options for AF include medication, AF suppression and surgery Medication or drug therapy is generally the first treatment option employed to control the rate at which the upper and lower chambers of the heart beat. Conventional medications used to treat AF include beta-blockers, such as metoprolol or propanolol, and calcium-channel blockers, such as verapamil or diltiazem, which depress conduction and prolong refractoriness in the AV node. Other medications such as amiodarone, ibutilide, dofetilide, propafenone, flecainide, procainamide, quinidine and sotalol are used to affect the electrophysiology of the heart to maintain normal sinus rhythm and can thereby terminate or, in some cases, prevent AF. Although anticoagulants or blood-thinners such as warfarin or aspirin are not designed to treat AF, these medications are often used to reduce the risk of clot formation and stroke which, as previously discussed, often occur in patient's suffering from AF.
AF suppression, frequently a second treatment option for patients with AF, may be accomplished using an implanted pacemaker to stimulate the heart in a way that preempts any irregular rhythms. In general, the pacemaker stimulates or overdrives the heart at a rate slightly higher than its normal, intrinsic rate. Overdriving the heart enables the device to control the heart rate and, thereby, suppress potential episodes of AF.
Another alternative treatment for AF is surgery. In general, an electrophysiology study is first performed to characterize the arrhythmic event. This study usually includes mapping the exact locations of the electrical impulses and conduction pathways along the cardiac chambers using conventional mapping techniques. After locating the cardiac tissue that is causing the arrhythmia, the tissue is then surgically altered or removed to prevent conduction of aberrant electrical impulses in the heart. One example of a surgical procedure used to treat cardiac arrhythmias is the Maze procedure.
The Maze procedure is an open-heart or percutaneous surgical procedure designed to interrupt the electrical patterns or conduction pathways responsible for cardiac arrhythmia. Originally developed by Dr. James L. Cox, the Maze procedure involves carefully forming a “maze” of surgical incisions (from which the procedure's name is derived) in both atria to prevent the formation and conduction of errant electrical Impulses, while still preserving the function of the atria. The incisions channel or direct the electrical impulses along the heart to maintain synchrony of contraction between the atria and ventricles of the heart, thereby producing a normal heartbeat, in addition, resulting scar tissue generated by the incisions also prevents formation and conduction of aberrant electrical signals that cause AF, thereby eradicating the arrhythmia altogether.
Although surgical intervention, such as the Maze procedure, has proven successful in treating AF, these procedures are highly invasive, generate many post-operative complications, require lengthy patient recovery times and are quite costly. As a result, minimally invasive ablation techniques have become more popular and have been offered as an alternative treatment to surgical intervention for patients suffering from AF.
Cardiac ablation techniques typically involve the removal or destruction of cardiac tissue and the electrical pathways that cause the abnormal heart rhythm. In general, cardiac ablation is less costly, has fewer side effects and requires less recovery time for the patient compared to more invasive procedures. There are various methods by which a cardiac ablation procedure may be performed. These methods and energy modalities include cryoablation, radiofrequency (RF) ablation, laser ablation, microwave, vaporization, balloon ablation, drug elution and photodynamic therapy.
During an ablation procedure, an electrophysiology study is first performed to characterize the arrhythmic event and map the precise locations that exhibit the arrhythmia. Once these sites are identified, an ablation catheter is maneuvered to each of these sites and a sufficient amount of energy is delivered to ablate the tissue. As a result, the energy destroys the targeted tissue and, thus, makes it incapable of producing or conducting arrhythmia, while leaving the adjacent healthy tissue intact and functional.
In addition to ablating the specific arrhythmic tissue sites, alternative ablation procedures, such as cardiac segmentation procedures, have been developed to mechanically isolate or re-direct errant electrical signals in the heart. These procedures typically involve forming one or more linear or curvilinear lesions in the wall tissue of the heart to segment the cardiac chambers, similar to the above-described Maze procedure. These segmented lesions are generally formed in the atrial tissue of the heart, although accessory pathways, such as those through the wall of an adjacent region along the coronary sinus, have also been produced.
Advances in mapping and characterizing cardiac arrhythmias, particularly AF, have provided much insight into the mechanism of AF. Research has shown that there are at least six different locations in the left and right atria of the heart where relatively large, circular waves of continuous electrical activity (i.e., macro reentrant circuits) occur in patients suffering from AF. Recently, it has been determined that these reentrant circuits or wavelets may actually be confined to a limited area near the pulmonary veins. In other words, some forms of AF may even be triggered or maintained by a single focus of automatic firing. As a result, several procedures have been developed whereby one or more ablation segments or lesions are formed in tissue to isolate the pulmonary veins and thereby block the electrical impulses that cause AF.
Although catheter based ablation procedures are less invasive than conventional surgical procedures, there are various complications that may occur. Examples of possible complications include ablation injuries, bleeding, hematoma, pericardial effusion and cardiac tamponade, failure of the procedure, scar formation and stenosis. In addition, the time course of lesion maturation and scar formation following cardiac ablation procedures often result in delayed onset of electrical isolation and high incidence of post-operative atrial fibrillation.
In view of the above, there is a need for a minimally invasive device and more effective and efficient methods to treat cardiac arrhythmias. In particular, it is desirable that the methods have a high success rate at treating arrhythmias, have minimal to no side-effects or related complications, and can be completed more rapidly than conventional methods. In addition, the treatment methods should also reduce patient recovery times and hospital costs. Overall, the method of treatment should also improve the quality of life for patients.